As it is known in the art, a fuel cell is an electrochemical device or apparatus suitable for producing an electromotive force by taking advantage of electron exchange occurring in Red-Ox reactions. The electric efficiency of an electrochemical device is measured by the ratio of chemical energy conversion, which is here defined as Gibbs free energy associated with an unitary molar amount of reagents involved in a Red-Ox reaction, which takes place in the electrochemical device in question, into electric power.
So far, it has been the general pursuit the reduction to a minimum of the production costs of a fuel cell, that are directly proportional to the active surface area of the involved electrodes, and such a need has directed the research efforts towards developing fuel cells with the highest possible ratio of electric power density supplied per unit of active surface area of the electrodes.
Moreover, the fuel cell development strategies of today, independently from the specific electrochemical processes taking place in the fuel cell, are based on the assumption that the internal dissipative resistances of an fuel cell are set by the technological characteristics of the apparatus itself, such as type of materials, catalysts, cell architecture, electrolytes and the thermodynamics of the reactions that take place in the apparatus. The same also applies to the electric efficiency. Furthermore, the provision of feeding flows of the reagents at almost constant pressure and concentration implies that the electric power density produced per unit of active surface area is also determined by the technological characteristics of the apparatus. This implies that, since the electric power density producible by a given piece of electrochemical technology is fixed, it is possible to determine the total electric power output simply by determining the total electrodes surface area involved in the reaction.
More particularly, independently of the used technology it is generally implicitly assumed that the relation between the current density produced per unit of active surface area (i.e. the electric power density produced per unit of electrodes active surface area) and the electrical efficiency of a fuel cell, is negligible or none. In other words, the established theory was that the fuel cell efficiency is independent from the electric current density obtained per unit of active surface area of the electrodes. This assumption together with the need of reducing the production costs of fuel cells has led to the pursuit of materials, catalysts, cell architectures, electrolytes and processes geared to obtaining the lowest possible internal resistance and the highest possible current density per unit of active surface area.
With reference to a fuel cell, “main Red-Ox reactions” are defined as the Red-Ox reactions involved in the production of an electric current flowing in an external circuit connected in parallel to the fuel cell. Any chemicals not involved in the main Red-Ox reactions are defined as “diluents”, whereas any chemicals actually involved in the main Red-Ox reactions are defined as “reagents”. Of course, diluents, here includes all the range of possible chemicals.